access to my computer - any posibility?

Please help me out.

I am using my Ex'computer after seperation. I turned off the guest account.
He must know the IP address and all of the systems except my log on password.

I am not sure if he has nstalled any program to access this computer.

How to check this out? is it possible for him to see my screen? I don't trust him. He is a software developer.I have no idea about computer.

Thanks,

WOU